Gas turbine and fuel injector for the same

ABSTRACT

A fuel injector for a gas turbine engine includes an injector housing having a central cavity configured to fluidly couple with a combustor of the gas turbine engine. The fuel injector may include a fuel nozzle at the upstream end of the central cavity. The fuel nozzle may be configured to direct a first fuel into the central cavity. The fuel injector may also include an annular air inlet disposed circumferentially about the fuel nozzle at the upstream end of the central cavity, and an annular air discharge outlet circumferentially disposed about the exit opening of the central cavity. The fuel injector may further include an annular fuel discharge outlet circumferentially disposed about the air discharge outlet. The fuel discharge outlet may be configured to discharge a second fuel into the combustor circumferentially around the air discharge outlet.

TECHNICAL FIELD

The present disclosure relates generally to a fuel injector for a gas turbine engine.

BACKGROUND

In a typical gas turbine engine (GTE), one or more fuel injectors direct a liquid or gaseous hydrocarbon fuel into a combustion chamber (called combustor) for combustion. The combustion of hydrocarbon fuels in the combustor produce undesirable exhaust constituents such as NOx. Different techniques are used to reduce the amount of NOx emitted by GTEs. In one technique, a lean premixed fuel-air mixture is directed to the combustor to burn at a relatively low combustion temperature. A low combustion temperature reduces NOx formation. In another technique, steam is directed to the combustor to reduce the temperature and reduce NOx production. U.S. Pat. No. 7,536,862 B2 to Held et al. (the '862 patent) describes a fuel injector for a gas turbine engine in which fuel is injected from the fuel injector into the combustor through primary and secondary openings. Steam is injected alongside the fuel to decrease the temperature of the flame in the combustor, and thereby reduce NOx production.

SUMMARY

In one aspect, a fuel injector for a gas turbine engine is disclosed. The fuel injector includes an injector housing including a central cavity extending along a longitudinal axis from an upstream end to a downstream end. The downstream end of the central cavity may include an exit opening configured to fluidly couple the central cavity to a combustor of the gas turbine engine. The fuel injector may include a fuel nozzle at the upstream end of the central cavity. The fuel nozzle may be configured to direct a first fuel into the central cavity. The fuel injector may also include an annular air inlet of the central cavity disposed circumferentially about the fuel nozzle at the upstream end of the central cavity, and an annular air discharge outlet circumferentially disposed about the exit opening of the central cavity. The fuel injector may further include an annular fuel discharge outlet circumferentially disposed about the air discharge outlet. The fuel discharge outlet may be configured to discharge a second fuel into the combustor circumferentially around the air discharge outlet.

In another aspect, a method of operating a gas turbine engine is disclosed. The method may include directing a premixed fuel-air mixture into a combustor of the gas turbine engine through a central cavity of a fuel injector. The premixed fuel-air mixture may be a mixture of a first fuel and a first quantity of compressed air. The central cavity may extend from a first end fluidly coupled to the combustor to a second end. The method may also include directing a second quantity of compressed air into the combustor circumferentially around the premixed fuel-air mixture. The method may further include increasing an angular velocity of a second fuel in the fuel injector, and directing the second fuel into the combustor circumferentially around the second quantity of compressed air.

In yet another aspect, a gas turbine engine is disclosed. The gas turbine engine may include a combustor system including a combustor, and a fuel injector extending from a first end to a second end. The fuel injector may be coupled to the combustor at the first end and may include a central cavity extending from the first end to the second end along a longitudinal axis. The central cavity may be configured to direct a premixed fuel-air mixture into the combustor. The fuel injector may include a fuel nozzle centrally located on the central cavity at the second end and may be configured to direct a gaseous fuel into the central cavity. The fuel injector may also include a first air discharge outlet circumferentially disposed around the fuel nozzle. The first air discharge outlet may be configured to direct a first quantity of compressed air into the central cavity to mix with the gaseous fuel and create the premixed fuel-air mixture in the central cavity. The fuel injector may also include a second air discharge outlet configured to direct a second quantity of compressed air into the combustor circumferentially around the premixed fuel-air mixture entering the combustor from the central cavity. The fuel injector may further include an outer passageway circumferentially disposed about the central cavity. The outer passageway may be configured to selectively direct a gaseous fuel and/or a liquid fuel into the combustor circumferentially around the second quantity of compressed air when operated in a co-firing mode or singly when operated in a more conventional mode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of an exemplary disclosed gas turbine engine system;

FIG. 2 is a perspective view of an exemplary fuel injector used in the turbine engine of FIG. 1;

FIG. 3 is a cross-sectional illustration of the fuel injector of FIG. 2; and

FIG. 4 is a flow chart that illustrates an exemplary operation of the fuel injector of FIG. 2.

DETAILED DESCRIPTION

FIG. 1 illustrates an exemplary gas turbine engine (GTE) 100. GTE 100 may have, among other systems, a compressor system 10, a combustor system 20, a turbine system 70, and an exhaust system 90 arranged along an engine axis 98. Compressor system 10 compresses air and delivers the compressed air to an enclosure 72 of combustor system 20. The compressed air is then directed from enclosure 72 into a combustor 50 through one or more fuel injectors 30 positioned therein. One or more types of fuel (such as, for example, a gaseous fuel and a liquid fuel) may be directed to the fuel injector 30 through fuel lines (not identified). GTE 100 may operate using different types of fuel depending upon availability of a particular fuel. For instance, when GTE 100 operates at a site with an abundant supply of a gaseous fuel (such as natural gas), the gaseous fuel may be used to operate the GTE 100. Under some operating conditions, another type of fuel (such as diesel fuel) may be used to operate the GTE 100. The fuel burns in combustor 50 to produce combustion gases at high pressure and temperature. These combustion gases are used in the turbine system 70 to produce mechanical power. Turbine system 70 extracts energy from these combustion gases, and directs the exhaust gases to the atmosphere through exhaust system 90. The layout of GTE 100 illustrated in FIG. 1, and described above, is only exemplary and fuel injectors 30 of the current disclosure may be used with any configuration and layout of GTE 100.

FIG. 2 is a perspective view of an embodiment of fuel injector 30 which may be coupled to combustor 50. FIG. 3 is a cross-sectional view of fuel injector 30 schematically illustrated as being coupled to combustor 50. In the description that follows, reference will be made to both FIGS. 2 and 3. Fuel injector 30 may be a single fuel injector or a dual fuel injector. A dual fuel injector is an injector that is configured to deliver different types of fuel (for example, gaseous and liquid fuel) to the combustor 50. Fuel injector 30 extends from a first end 12 to a second end 14 along a longitudinal axis 88. As illustrated in FIG. 2, the fuel injector 30 may have a shape resembling the frustum of a cone proximate the first end 12. The first end 12 of the fuel injector 30 may be coupled to combustor 50, and the second end 14 of the fuel injector 30 may extend into enclosure 72 (see FIG. 1). As is known in the art, combustor 50 is an annular chamber, bounded by a liner 52, located around engine axis 98 of GTE 100 (see FIG. 1).

Compressed air from enclosure 72 enters fuel injector 30 through one or more inlet openings 16 a and 18 a at the second end 14 of fuel injector 30. In some embodiments, these inlet openings may be ring-shaped openings annularly positioned around longitudinal axis 88. However inlet openings of other shapes are also contemplated. For instance, in some embodiments, inlet openings 16 a and 18 a may resemble segments of a circle around longitudinal axis 88. Although inlet openings 16 a and 18 a may have any size, the area of inlet opening 16 a (which is positioned radially closer to longitudinal axis 88) will be larger than the area of inlet opening 18 a positioned further away from longitudinal axis 88. Because of this larger opening area, the quantity (volume/time, mass flow rate, etc.) of air entering the fuel injector 30 through inlet opening 16 a will be larger than that entering through inlet opening 18 a. At the second end 14, one or both of inlet openings 16 a and 18 a may include features (angles, chamfers, etc.) configured to modify the angle of entry of air into the fuel injector 30. In some embodiments, the inlet openings 16 a and 18 a may be configured such that the flow of air into the fuel injector 30 through these inlet openings 16 a, 18 a is substantially axial (that is, along the longitudinal axis 88).

Compressed air that enters through inlet opening 16 a flows through a central cavity 16 of the fuel injector 30. Central cavity 16 is a centrally located passageway that extends along the longitudinal axis 88 from the inlet opening 16 a at the second end 14 to an exit opening 16 b at the first end 12. The exit opening 16 b directs the compressed air in the central cavity 16 into the combustor 50. Exit opening 16 b may be centrally positioned at the first end 12 of the fuel injector 30 around the longitudinal axis 88. In some embodiments, the central cavity 16 may be cylindrically shaped and have a substantially constant diameter from the first end 12 to the second end 14. However, in some embodiments, the central cavity 16 may have a generally convergent shape such that the diameter of the central cavity 16 at the first end 12 is smaller than the diameter at the second end 14. In some embodiments, the central cavity 16 may converge substantially uniformly along an entire length of the fuel injector 30. However in some embodiments, the central cavity 16 may only converge along a portion of its length. For example, only a portion of the length of the central cavity 16 proximate first end 12 may be convergent while the remaining portion (that is, proximate the second end 14) of the central cavity 16 may be substantially cylindrical. The angle of convergence may depend upon the application. In some embodiments, the angle of convergence may be such that the diameter of the central cavity 16 at the first end 12 is 2-3% smaller than its diameter at the second end 14. A convergent central cavity 16 increases the velocity of the compressed air as it flows therethrough.

A fuel nozzle 26 may be positioned at the second end 14 of the central cavity 16 to direct a fuel into the central cavity 16. A fuel pipe 24 may direct the fuel into the fuel nozzle 26. In general, fuel pipe 24 and the fuel nozzle 26 may direct any type of fuel into the central cavity 16. In some embodiments a gaseous fuel may be directed into the central cavity 16 through the fuel nozzle 26. In some embodiments, this gaseous fuel may be a high calorific fuel gas (such as, for example, natural gas, oil well gas, coal gas, etc.). This fuel may mix with compressed air entering the central cavity 16 through the inlet opening 16 a and create a premixed fuel-air mixture in central cavity 16. The premixed fuel-air mixture travels downstream and enters the combustor 50 through exit opening 16 b to undergo combustion. In embodiments where the central cavity 16 is convergent, the linear velocity of the fuel-air mixture increases as it travels towards the convergent portion. The increased linear velocity forces the ignited fuel-air mixture away from the fuel injector 30 and thereby assists in reducing flashback.

Compressed air that enters the fuel injector 30 through inlet opening 18 a flows through an inner air passage 18 and enters the combustor 50 through an exit opening 18 b at the first end 12. Exit opening 18 b of the inner air passage 18 is an annularly shaped opening positioned radially outwards of exit opening 16 b of the central cavity 16. Inner air passage 18 is an annular passageway symmetrically disposed about the longitudinal axis 88, and positioned radially outwards of the central passageway 16. The compressed air from the inner air passage 18 flows into the combustor 50 around the premixed fuel-air mixture that enters the combustor 50 from the central cavity 16. At the outlet of fuel injector 30, the compressed air from the inner air passage 18 acts as a shroud around the premixed fuel-air mixture from the central cavity 16. The relative size of the inlet openings 16 a and 18 a may be such that the quantity of air entering the combustor 50 through the inner air passage 18 is sufficient to act as a shroud around the premixed fuel-air mixture (from the central cavity 16) without diluting the concentration of the fuel in the fuel-air mixture. The shape of the inner air passage 18 may also be configured to reduce the mixing of the air from the inner air passage 18 with the premixed fuel-air mixture from the central cavity 16.

Because of the generally conical shape of the fuel injector 30 proximate the first end 12, the inner air passage 18 may progressively converge towards the longitudinal axis 88 as it approaches the exit opening 18 b. That is, the radial distance of the inner air passage 18 from the longitudinal axis 88 may decrease as the inner air passage 18 extends towards the exit opening 18 b. In some embodiments, as illustrated in FIG. 3, only a portion of the length of the inner air passage 18, proximate the first end 12, may have a convergent shape. However, it is contemplated that in some embodiments, substantially an entire length of the inner air passage 18 (from the second end 14 to the first end 12) may be convergent. The gradually decreasing radial distance of the inner air passage 18 will decrease the cross-sectional area of the passage as it approaches the exit opening 18 b. The decreasing cross-sectional area will increase the linear velocity of the compressed air in the inner air passage 18 as it moves towards the exit opening 18 b. The decreasing radial distance will increase the spin or the angular velocity of the compressed air in the inner air passage 18 as it travels towards the exit opening 18 b. Because of the principle of conservation of angular momentum, the compressed air exiting the exit opening 18 b with increased angular velocity will move outwardly in a direction away from the longitudinal axis 88. The convergent shape of the inner air passage 18 thus reduces the tendency of the compressed air from the inner air passage 18 to mix with, and dilute, the premixed fuel-air mixture from the central cavity 16 immediately upon exit into the combustor 50. It should be noted that a convergent shape of the inner air passage 18 is not a requirement, and in some embodiments, the inner air passage 18 may not be convergent. In some embodiments, inner air passage 18 may include swirler vanes 22 positioned thereon. These swirler vanes 22 may impart a swirl to the compressed air as it travels towards the combustor 50.

Fuel injector 30 also includes an annularly shaped outer passage 32 disposed radially outwards of the inner air passage 18. The outer passage 32 may extend from an inlet opening 32 a proximate the second end 14 to an annularly shaped exit opening 32 b positioned radially outwards exit opening 18 b of inner air passage 18. The inlet opening 32 a may open into an annular chamber 34 disposed at the second end 14 of the fuel injector 30. Annular chamber 34 may be an annular cavity that extends around the fuel injector 30 at the second end 14. The annular chamber 34 may include multiple inlet ports (with fluid conduits 36 coupled thereto) to direct one or more fluids into the annular chamber 34. In some embodiments, these multiple inlet ports may include a first inlet port 34 a, a second inlet port 34 b, a third inlet port 34 c, and a fourth inlet port 34 d. The first inlet port 34 a may be configured to deliver a gaseous fuel, a second inlet port 34 b may be configured to direct a liquid fuel, a third inlet port 34 c may be configured to direct shop air, and a fourth inlet port 34 d may be configured to direct steam (or water) into the annular chamber 34. During operation of GTE 100, one or more fluids may be selectively directed into the annular chamber 34 through these multiple inlet ports at the same time. For example, in some applications a liquid fuel and shop air may be directed into the annular chamber 34, at the same time, during starting of the GTE 100. After GTE 100 reaches a desired speed, the liquid fuel and shop air supply may be stopped, and gaseous fuel may be directed into the annular chamber 34. The fluid (liquid fuel, gaseous fuel, shop air, steam, etc.) in the annular chamber 34 may travel through the outer passage 32 and enter the combustor 50 through exit opening 32 b.

Compressed air from enclosure 72 also enters the combustor 50 through an air swirler 28 positioned circumferentially outwardly of the fuel injector 30 at the first end 12. Air swirler 28 may include one or more blades or vanes shaped to induce a swirl to the compressed air passing therethough. Although the air swirler 28 illustrated in FIG. 3 is an axial air swirler, any type of air swirler known in the art (for example, radial air swirler) may be used. As the compressed air from the enclosure 72 flows into the combustor 50 through the air swirler 28, a swirl will be induced to the air. This swirled air will spin outwardly and move towards the outer walls of combustor 50. Since air swirlers and their role in the functioning of GTE 100 are known in the art, for the sake of brevity, air swirler 28 is not discussed in detail herein.

In some embodiments, a portion of the length (or even the entire length) of the outer passage 32 may converge towards the longitudinal axis 88 as it approaches the exit opening 32 b. That is, the radial distance (and hence the cross-sectional area) of the outer passage 32 from the longitudinal axis 88 may decrease towards the combustor 50. As explained earlier with reference to the inner air passage 18, this decreasing radial distance increases the linear and angular velocity of the fluid as it travels through the outer passage 32. Due to the increased angular velocity, the fluid exiting the exit opening 32 b will spin outwardly and move in a direction away from the longitudinal axis 88 (because of conservation of angular momentum). This outwardly moving fluid will meet and mix with the swirled air stream from the air swirler 28 and rapidly mix. When the fluid directed through the outer passage 32 is a fuel (liquid or gaseous), the mixing of the fuel and air reduces the flame temperature, and thereby the NO production, in the combustor 50. The angle of convergence (the angle between the outer passage 32 and the longitudinal axis 88) of the outer passage 32 may be any value and may depend upon the application. In some exemplary embodiments, an angle of convergence of between about 20° and 80° may be suitable. It should be noted that, although FIG. 3 illustrates the thickness of the convergent outer passage 32 and the convergent inner air passage 18 as decreasing towards the first end 12, this is not a requirement. That is, in some embodiments, a convergent passage (outer passage 32 and/or inner air passage 18) may be a passageway with a constant thickness along its length that angles towards the longitudinal axis 88.

In some embodiments, some or all of the multiple ports (first, second, third, and fourth port 34 a, 34 b, 34 c, 34 d) may be positioned in annular chamber 34 such that the fluids enter the annular chamber 34 tangentially to induce a spin to the fluid. The induced spin may assist in thorough mixing of the fluid with gases in the combustor 50. A fluid may be tangentially directed into the annular chamber 34 by tangentially positioning a port or by adapting the shape of the port (for example, a curved port, angled port, etc.) for tangential entry. Although a cylindrically shaped annular chamber 34 is illustrated in FIGS. 2 and 3, in some embodiments, annular chamber 34 may be a toroidal (snail shell shaped) cavity in which the area of the cavity decreases with distance around the longitudinal axis 88. In such an embodiment, as a fluid enters the toroidal annular chamber 34 and travels around the gradually narrowing cavity, a spin is introduced to the fluid.

Although the annular chamber 34 is illustrated as having four inlet ports, this is only exemplary. Other embodiments of fuel injectors 30 may have a different number of inlet ports. For example, in some embodiments of fuel injector 30, only one inlet port may be provided to direct a gaseous fuel or a liquid fuel into the annular chamber 34, and in another embodiment two inlet ports may be provided to direct a liquid fuel and shop air into the annular chamber 34. Any type of gaseous fuel (natural gas, coal gas, etc.) and liquid fuel (for example, kerosene, diesel fuel, etc.) may be directed into the annular chamber 34 through the first and second ports 34 a, 34 b, respectively. In some embodiments, the same gaseous fuel may be delivered through the first port 34 a and the fuel nozzle 26, while in other embodiments, different gaseous fuels may be provided through the first port 34 a and the fuel nozzle 26. Third port 34 c may direct shop air to the annular chamber 34. The shop air may be air compressed using a different compressor than that compressor system 10 of the GTE 100. In some embodiments, shop air may be directed to the combustor 50 only during lightoff of the GTE 100. During lightoff, the shop air may have a higher pressure than the compressed air of the compressor system 10. The shop air may assist in atomization of the liquid fuel when liquid fuel is directed into the annular chamber 34. The steam directed into the annular chamber 34 through the fourth port 34 d may assist in reducing the flame temperature (and thereby reduce NO production) in the combustor 50.

A common concern with fuel injectors is the cross-contamination of fuel delivery lines during operation. During operation, combustion driven turbulent pressure fluctuations may induce small pressure variations in the vicinity of different fuel injectors 30 in the combustor 50. These pressure differences may induce fuel to migrate into fuel lines in lower pressure regions and create carbonaceous deposits therein. For example, when GTE 100 operates with liquid fuel delivered through outer passage 32, the central cavity 16 may only direct compressed air (from inlet opening 16 a) to the combustor 50. Absent the compressed air supply through exit opening 18 b that forms a shroud (or an air shell, air curtain, etc.) around exit opening 16 b, pressure fluctuations in the combustor 50 may cause the liquid fuel to enter the central cavity 16 (and the liquid fuel nozzle 26) and ignite or decompose therein to cause coking. However, the compressed air supply through outlet opening 18 b circumferentially disposed around outlet opening 16 b prevents the liquid fuel from migrating into the central cavity 16. The increased angular momentum of the liquid fuel emanating from the outlet opening 32 b of the outer passage 32 will also cause the liquid fuel to move in a direction away from the longitudinal axis 88 and assist in keeping the liquid fuel away from the central cavity 16. In a similar manner, the compressed air supply through the outlet opening 18 b shrouds and prevents the premixed fuel-air mixture from the central cavity 16 from entering and depositing in the outer passage 32.

INDUSTRIAL APPLICABILITY

The disclosed fuel injector may be applicable to any turbine engine. In one embodiment of the fuel injector, two separate streams of fuel are directed into the combustor through the fuel injector, and the respective fuel outlets are positioned to reduce cross-contamination. A compressed air stream is configured to separate the two fuel outlets from each other. In some embodiments, the fuel through the fuel outlets is directed to the combustor in a manner to reduce flashback. The operation of a gas turbine engine with an embodiment of a disclosed fuel injector will now be described.

FIG. 4 is a flowchart that illustrates an exemplary application of fuel injector 30. GTE 100 may be started with a liquid fuel directed to combustor 50 through outer passage 32, and transitioned to a gaseous fuel directed to combustor 50 through central cavity 16 at a nominal power. During startup, compressed air from enclosure 72 is directed into the combustor 50 through the air swirler 28 and through one or more fuel injectors 30 coupled to the combustor 50 (step 110). The compressed air supplied though each fuel injector 30 flows through the central cavity 16 and the inner air passage 18 of the fuel injector 30. In the current application, the total amount of compressed air that is directed into the combustor 50 through the fuel injectors 30 and the air swirler 28 is termed as “injection air.” In a typical GTE 100, about 15-25% of the total air directed to the combustor 50 is injection air. The remaining amount of compressed air (that is, 75-85%) enters the combustor 50 through other paths (for instance, as cooling air supply through perforations on the liner 52, etc.). In an exemplary embodiment of the fuel injector 30, about 15% of the injection air enters the combustor 50 through the fuel injectors 30, and the remaining amount (about 85%) enters the combustor 50 through the air swirler 28. Of the portion of the injection air that enters the combustor 50 through the fuel injectors 30 (about 15% of the injection air), about 5% flows through the central cavity 16, and the remaining amount (about 10%) flows through the outer air passage 18. That is, in such an exemplary embodiment of fuel injector 30, the amount (volume/time, flow rate, etc.) of compressed air flowing through the inner air passage 18 is about 2 (that is, about 10%/about 5%) times higher than the amount of air flowing in through the central cavity 16. In general, the amount of compressed air flowing through the inner air passage 18 may be between about 1.5 and 4 times the amount of air flowing in through the central cavity 16. The compressed air exiting into the combustor 50 from the inner air passage 18 surrounds the exit opening 16 b of the central cavity 16, and acts as a shroud around the compressed air from the central cavity 16.

Liquid fuel is also directed into the combustor 50, around the compressed air supply from the inner air passage 18, through outer passage 32 (step 120). In some embodiments, due to the shape of the outer passage 32 that directs the liquid fuel to the combustor 50, the angular velocity and the linear velocity of the liquid fuel may increase as the fuel travels towards the combustor 50. The increased angular velocity may cause the liquid fuel that exits into the combustor 50 to be flung outwards towards the combustor walls and away from the central cavity 16. The outwardly traveling liquid fuel may reduce the possibility of the liquid fuel migrating into the central cavity 16 and decomposing therein. The compressed air supply from the inner air passage 18 may also act as an air curtain that prevents the liquid fuel from migrating into the central cavity 16.

Within the combustor 50, the outwardly moving liquid fuel stream will mix with the portion of injection air flowing into the combustor 50 through the air swirler 28 (step 130). The mixed liquid fuel and air will ignite and travel outwards towards the combustion walls and spread around the combustor 50 (step 140). The GTE 100 is then accelerated to a desired power value (idle speed, a nominal load, etc.) using the liquid fuel (step 150). After the desired power value is reached, gaseous fuel may be injected into the central cavity 16 through the fuel nozzle 26 (step 160). This gaseous fuel mixes with the portion of the injection air that flows through the central cavity 16 and creates a premixed fuel-air mixture (step 170). This premixed fuel-air mixture enters the combustor 50, shrouded by the compressed air supply from the circumferentially disposed exit opening 18 b (step 180). Within the combustor, the premixed fuel-air mixture ignites (step 190).

The liquid fuel supply through the outer passage 32 may now be stopped (step 200). The compressed air stream surrounding the premixed fuel-air mixture from the central cavity 16 prevents the fuel-air mixture from migrating upwards into the outer passage 18 and decomposing therein. In some embodiments, the shape of the central cavity 16 may be configured to increase the linear velocity of the premixed fuel-air mixture entering the combustor 50. The increased linear velocity of the fuel-air mixture assists in moving the ignited mixture away from the fuel injector 30 and reducing the possibility of flashback. In some embodiments, after terminating the liquid fuel supply via outer passage 32, gaseous fuel may be supplied to the combustor 50 through the outer passage 32. In some embodiments, when the flame temperature within the combustor 50 causes the NOx emissions to increase above a desired value, steam may be directed into the combustor 50 through the outer passage 32 to reduce the flame temperature. In some embodiments, along with the liquid fuel, shop air may also be directed into the combustor 50 through the outer air passage 32 to provide additional air for combustion. The ability to direct multiple fuels and other fluids into the combustor 50 through the fuel injector 30 increases the versatility of the fuel injector 40 while reducing NOx emissions.

It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed fuel injector. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed fuel injector. It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents. 

What is claimed is:
 1. A fuel injector for a gas turbine engine comprising: an injector housing including a central cavity extending along a longitudinal axis from an upstream end to a downstream end, the downstream end of the central cavity including an exit opening configured to fluidly couple the central cavity to a combustor of the gas turbine engine; a fuel nozzle at the upstream end of the central cavity, the fuel nozzle being configured to direct a first fuel into the central cavity; an annular air inlet of the central cavity disposed circumferentially about the fuel nozzle at the upstream end of the central cavity; an annular air discharge outlet circumferentially disposed about the exit opening of the central cavity; and an annular fuel discharge outlet circumferentially disposed about the air discharge outlet, the fuel discharge outlet being configured to discharge a second fuel into the combustor circumferentially around the air discharge outlet.
 2. The fuel injector of claim 1, further including an annular inner passageway extending from an inlet opening at the upstream end to the annular air discharge outlet at the downstream end, the inner passageway being disposed radially outwards of and symmetrically about the central cavity.
 3. The fuel injector of claim 2, wherein at least a portion of the inner passageway forms a convergent passageway.
 4. The fuel injector of claim 2, further including an annular outer passageway extending from the upstream end to the fuel discharge outlet at the downstream end, the outer passageway being disposed symmetrically about the central cavity and radially outwards the inner passageway.
 5. The fuel injector of claim 4, wherein at least a portion of the outer passageway forms a convergent passageway.
 6. The fuel injector of claim 4, further including an annular chamber extending around the upstream end of the injector housing, wherein the outer passageway is fluidly coupled to the annular chamber at the upstream end.
 7. The fuel injector of claim 6, further including a plurality of inlet ports coupled to the annular chamber, wherein a first inlet port of the plurality of inlet ports is configured to direct the second fuel into the outer passageway.
 8. The fuel injector of claim 7, wherein the second fuel is a liquid fuel and the plurality of inlet ports includes a second inlet port configured to direct a gaseous fuel into the outer passageway.
 9. The fuel injector of claim 8, wherein the plurality of inlet ports includes a third inlet port configured to direct steam into the outer passageway.
 10. The fuel injector of claim 1, wherein at least a portion of the central cavity is a convergent passageway.
 11. A method of operating a gas turbine engine comprising: directing a premixed fuel-air mixture into a combustor of the gas turbine engine through a central cavity of a fuel injector, the premixed fuel-air mixture being a mixture of a first fuel and a first quantity of compressed air, the central cavity extending from an upstream end to a downstream end fluidly coupled to the combustor; directing a second quantity of compressed air into the combustor circumferentially around the premixed fuel-air mixture; and directing a second fuel into the combustor circumferentially around the second quantity of compressed air, wherein directing the second fuel includes increasing an angular velocity of the second fuel in the fuel injector.
 14. The method of claim 13, wherein directing the premixed fuel-air mixture includes directing the first fuel into the upstream end of the central cavity, and directing the first quantity of compressed air into the central cavity circumferentially around the first fuel.
 15. The method of claim 13, further including increasing an angular velocity and a linear velocity of the second quantity of compressed air in the fuel injector prior to directing the second quantity of compressed air into the combustor.
 16. The method of claim 13, further including directing steam into the combustor circumferentially around the second quantity of compressed air.
 17. A gas turbine engine, comprising: a combustion system including a combustor; and a fuel injector extending from an upstream end to a downstream end along a longitudinal axis, the fuel injector being coupled to the combustor at the downstream end, the fuel injector including: a central cavity extending from the upstream end to a central opening at the downstream end, the central cavity being configured to direct a fuel into the combustor through the central opening; an annular air discharge outlet circumferentially disposed about the central opening, the annular air discharge outlet being configured to direct compressed air into the combustor circumferentially around the fuel entering the combustor from the central cavity; and an outer passageway circumferentially disposed about the central cavity, the outer passageway being configured to selectively direct a gaseous fuel and a liquid fuel into the combustor circumferentially around the compressed air from the air discharge outlet.
 18. The gas turbine of claim 17, wherein the outer passageway is fluidly coupled to an annular chamber that extends around the fuel injector at the upstream end, the annular chamber including a plurality of inlet ports coupled thereto, the plurality of inlet ports including a first inlet port configured to direct the gaseous fuel into the outer passageway, a second inlet port configured to direct the liquid fuel into the outer passageway, and a third inlet port configured to direct steam into the outer passageway.
 19. The gas turbine of claim 17, wherein at least a portion of the central cavity is a convergent passageway.
 20. The gas turbine engine of claim 17, wherein at least a portion of the outer passageway is a convergent passageway. 